1. Field of the Invention
The present invention relates to a long thermal head composed by combining plural electric insulating substrates.
2. Description of the Prior Art
As the thermal head for use in thermal printing, the function of thermally recording as line printer in recording paper of JIS A1 or A0 format is required. It is at the present technically difficult to compose such long thermal head from a single head substrate made of ceramics such as electric insulating material, and it is therefore attempted to compose a long thermal head by mutually joining a plurality of head substrates.
FIG. 1 is a plan view of a typical conventional long thermal head 1. The thermal head 1 is composed of, for example, three head substrates 3a, 3b, 3c having heating resistance element rows 2a, 2b, 2c (collectively indicated by reference number 2) formed on the principal plane thereof respectively, which are mutually joined at junction positions 4a, 4b along the arranging direction of the heating resistance element row 2, so that the individual heating resistance element rows 2a, 2b, 2c may form a straight line.
FIG. 2 is a sectional view of the thermal head 1, and FIG. 3 is a magnified plane view near the junction position 4a, for example, of the thermal head 1. In the thermal head 1, cooling plates 10a, 10b of metal material are mounted on a support plate 9 made of metal material, and the head substrates 3a, 3b are mounted on the cooling plates 10a, 10b. On the head substrates 3a, 3b, glazed layers 11a, 11b made of, for example, glass are formed, and a heating resistance element layer (not shown) and common electrode 6 and individual electrode 7 are formed thereon, thereby forming a heating resistance element 5, so that thermal recording is effected on a thermal recording paper 14 against a platen roller 13.
On the head substrates 3a, 3b, at a mutual gap g1, plural heating resistance elements 5 are linearly formed respectively. At one common side of the heating resistance element rows 2a, 2b, at the head substrates 3a, 3b, the common electrode 6 commonly connected to the heating resistance element is formed, and on the opposite side of the common electrode 6 across the heating resistance elements 5, individual electrodes 7 individually connected to the respective heating resistance elements 5 are formed.
In such long thermal head 1, when the gap g2 of the heating resistance elements 5a, 5b at the remotest position along the arranging direction on the head substrates 3a, 3b, 3c becomes about 1/3, for example, of the arranging pitch g3 of the heating resistance elements 5, an unprinted at the time of thermal printing occurs at the junction position 4a (white-out). Accordingly, in order to shorten the gap g2 of the heating resistance elements 5a, 5b at the remotest position as far as possible, in this prior art, the shape of the common electrodes 6 on the head substrates 3a, 3b is warped so that the heating resistance elements 5a, 5b at the remotest position may approach toward the end portions 8a, 8b mutually joined to the head substrates 3a, 3b. The same holds true with the individual electrodes 7. In this way, it is attempted to prevent formation of white stripe in thermal printing between the heating resistance elements 5a, 5 b at the remotest position.
In this prior art, although the white stripe in thermal printing is prevented by approaching the heating resistance elements 5a, 5b at the remotest position, a step difference 12 of height d1 may be formed between the glazed layers 11a, 11b at the junction position 4a due to fluctuations of the machining precision and manufacturing thickness or other dimensional precision of the glazed layers 11a, 11b, head substrates 3a, 3b and cooling plates 10a, 10b on the head substrates 3a, 3b.
As shown in FIG. 3, when the distance g4 between the heating resistance element 5a at the remotest position of the head substrate 3a and the end portion 8a is 5 to 10 .mu.m and the gap relation is g1 g2 15 to 20 .mu.m, if the height d1 of the step difference 12 is 3 to 5 .mu.m, a white stripe of 50 to 70 .mu.m in width is formed, and if the height d1 is 20 .mu.m, a white stripe of 70 to 120 .mu.m in width was confirmed. Such white stripe significantly lowers the printing quality.
By placing a metal foil or other spacer between the cooling plates 10a, 10b and the support plate 9, the height d1 of the step difference 12 may be controlled to about 3 to 5 .mu.m, but it was known that a white stripe of 50 to 70 .mu.m in width was formed at g2 of 20 .mu.m if d1 was 5 .mu.m. Due to such step difference 12, moreover, the heating resistance elements 5 including the heating resistance elements 5a, 5b at the remotest position and their vicinity may be pushed to the head substrates 3a, 3b by the platen roller 13, and may fail to contact with the thermal recording paper 14. When power is supplied to such heating resistance elements 5 not contacting with the thermal recording paper 14, the temperature of the heating resistance elements 5 is excessively raised, and the preset resistance of the heating resistance elements 5 may fluctuate, or the resistance elements may be broken, thereby shortening the service life of the thermal head 1.
Besides, as a second prior art intended to solve the problem of formation of white stripe in thermal printing by bringing the heating resistance element at the remotest position at the junction closer in the heating element resistance arranging direction, a thermal head 1a shown in FIG. 4 is known. This thermal head 1a, same as in the foregoing prior art, is composed by mutually joining, for example, three head substrates 3a to 3c to compose a long thermal head 1a. End portions 17a, 17b, 17c of the head substrates 3a, 3c are formed as being inclined obliquely to the heating resistance element rows 2a to 2c, and the head substrates 3a to 3c are formed in a trapezoidal profile in a plan view. The heating resistance element rows 2a to 2c of the head substrates 3a to 3c are composed across a gap y along the subscanning direction. At this time, moreover, the end portions 17a to 17b are composed across a gap g5.
In such second prior art, by shortening the distance g2 of the heating resistance elements 5a to 5c at the remotest position, it is intended to prevent formation of white stripe in thermal printing at the junction positions 4a, 4b.
However, in such second prior art, too, as explained by reference to FIG. 1, the step difference 12 at the junction positions 4a, 4b cannot be avoided, and the same white stripe is formed to lower the printing quality.
Still more, in this prior art, the head substrates 3a to 3c are formed in a trapezoidal profile in a plan view. For example, when obliquely cutting the both end of the head substrate 3a, as shown in FIG. 5 (1), it requires a first positioning member 18 for supporting one end in the widthwise direction of the head substrate 3a , a cutting member 19 for cutting or dicing, opposite to the head substrate 3, forming a specific angle in the arranging direction of the heating resistance element row 2a, and a second positioning member 20, with a gap L1 spaced from the first positioning member 18, for positioning when cutting obliquely with the cutting member 19.
On the other hand, when cutting the other end of the head substrate 3a, the head substrate 3a is rotated a half revolution as indicated by arrow in FIG. 8, and it also requires first and second positioning members 18a, 20a disposed at a narrower gap L2 than the gap L1 between the above first and second positioning members 18, 20, and a cutting member 19a opposite the head substrate 3a at a different angle from the cutting member 19. Thus, the composition for cutting becomes complicated.
FIG. 6 is a plan view explaining other problem of the conventional thermal head 1a. The thermal head 1a is formed in an extended length, same as in the first prior art, by mutually joining, for example, three head substrates 3a to 3c, and the head substrate 3a has an external wiring substrate 15 possessing a connector 16 for exchanging signals for thermal printing with an external device, connected at one side of the heating resistance element 5a. That is, the individual electrodes 7 in the foregoing prior art are formed from the heating resistance element rows 2a toward the external wiring substrate 15. The end portion 8a of the head substrate 3a is processed in a shape to obliquely intersect with the arranging direction of the heating resistance element row 2a.
In the head substrate 3b joined to the head substrate 3a, concerning the heating resistance element rows 2b, the external wiring substrate 15 is connected on the opposite side of the connecting direction of the external wiring substrate 15 of the head substrate 3a. In the head substrate 3a joined to the head substrate 3b, the external wiring substrate 15 is connected in the opposite direction of the connecting direction of the external wiring substrate 15 in the head substrate 3b.
In this prior art, since the external wiring substrate 15 and connector 16 are connected mutually in reverse directions in every head substrate 3, and the width in the vertical direction of the thermal head 1a in FIG. 5 is extended and the structure becomes larger, and still more when the thermal head 1a is formed as a thermal printer, it is necessary to wire connecting leads at both sides in this widthwise direction of the thermal head 1a, and the wiring is complicated, and moreover the structure for leading the recording paper 14 into the thermal head 1a and the structure of discharging from the thermal head 1a are complicated.
As a different prior art intended to solve the problems due to the trapezoidal forming of the head substrates 3a to 3c, as shown in FIG. 6, it may be considered to cut off the end portions 17a to 17b of the head substrate 3a by inclining in the same direction, but in this case, the heating resistance element rows 2a to 2c are sequentially deviated in the widthwise direction in every one of the head substrate 3a to 3c, and it is necessary to set the contact width L3 with the platen roller commonly including the heating resistance element rows 2a to 2c relatively larger. That is, the size of the platen roller 13 is increased.